Observations of near-inertial current variability on the New England shelf

Observations from the Coastal Mixing and Optics (CMO) moored array (deployed from August 1996 through June 1997) and supplemental moored observations are used to describe near-inertial current variability over the New England shelf. Near-inertial band current variance comprises 10–20% of the total observed current variance, and has episodic peak speeds exceeding 30 cm/s. Near-inertial current variability during CMO is characterized by a first baroclinic mode vertical structure with one zero-crossing between 15 and 50 m. The zero-crossing is shallower during periods of stronger stratification. Laterally, near-inertial variability is coherent over the extent of the CMO moored array, and cross-shelf decorrelation scales for near-inertial currents are about 100 km, approximately the entire shelf width. The magnitude of near-surface near-inertial variability is stronger in the summer and weaker in the winter, following the seasonal variation in stratification and opposite the seasonal cycle in wind stress variance. During CMO, near-surface near-inertial kinetic energy is inversely related to surface mixed layer depth. Near-inertial variance decreases onshore, matching approximately the cross-shelf decrease in near-inertial energy predicted by a two-dimensional, linear, flat-bottom, twolayer, coastal wall model. In this model, the nullifying effects of a baroclinic wave emanating from the coastal wall play a dominant role in controlling the onshore decrease. Finally, strong persistent anticyclonic relative vorticity shifts near-inertial variability on the New England shelf to subinertial frequencies.Keywords: Inertial waves, Internal waves, Continental shelf processes, Analytical modelin

Heat and salt balances over the New England continental shelf, August 1996 to June 1997

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): C07017, doi:10.1029/2009JC006073.Heat and salt balances over the New England shelf are examined using 10 month time series of currents, temperature, and salinity from a four element moored array and surface heat and freshwater fluxes from a meteorological buoy. A principal result is closure of the heat budget to 10 W m−2. The seasonal variation in depth-average temperature, from 14°C in September to 5°C in March, was primarily due to the seasonal variation in surface heat flux and a heat loss in winter caused by along-shelf advection of colder water from the northeast. Conductivity sensor drifts precluded closing the salt balance on time scales of months or longer. For time scales of days to weeks, depth-average temperature and salinity variability were primarily due to advection. Advective heat and salt flux divergences were strongest and most complex in winter, when there were large cross-shelf temperature and salinity gradients at the site due to the shelf-slope front that separates cooler, fresher shelf water from warmer, saltier slope water. Onshore flow of warm, salty slope water near the bottom and offshore flow of cooler, fresher shelf water due to persistent eastward (upwelling-favorable) winds caused a temperature increase of nearly 3°C and a salinity increase of 0.8 in winter. Along-shelf barotropic tidal currents caused a temperature decrease of 1.5°C and a salinity decrease of 0.7. Wave-driven Stokes drift caused a temperature increase of 0.5°C and a salinity increase of 0.4 from mid December to January when there were large waves and large near-surface cross-shelf temperature and salinity gradients.The field program was funded by the Office of Naval Research, Code 322, under grant N00014‐95‐1‐0339. Analysis was also partially supported by the National Science Foundation Physical Oceanography program under grants OCE‐0647050 and OCE‐0548961

Dynamics of mean and subtidal flow on the New England shelf

Current and hydrographic observations from the Coastal Mixing and Optics experiment moored array, deployed from August 1996 through June 1997, are used to describe the velocity variability and evaluate the dynamics of circulation over the New England shelf on timescales ranging from a few days to several months. Subtidal (days to weeks) current variability was polarized along‐isobath and dominated by episodic bursts of westward flow. The along‐isobath subtidal flow was primarily geostrophic and barotropic, and was correlated with large‐scale along‐coast wind stress fluctuations oriented 45°T (65° counterclockwise from the local isobath orientation). Subtidal near‐surface ageostrophic transport matched estimates of wind‐driven Ekman transport; however, near‐bottom ageostrophic transport was much larger than estimates of Ekman transport from bottom stress. Low‐frequency (monthly and longer timescales) flow was generally westward and off‐shelf at all sites and depths, with the strongest westward flow during the fall. Low‐frequency along‐isobath currents were primarily geostrophic with baroclinic and barotropic components of similar magnitude. Depth‐averaged ageostrophic transport was quantitatively consistent with Ekman transport from wind and bottom stress. Measured bottom stress at both subtidal and low‐frequency timescales was weak, nearly an order of magnitude smaller than the wind stress. Low‐frequency fluctuations in the predominantly geostrophic along‐isobath flow were attributable to variations in the cross‐shelf density field associated with the seasonal cycle in surface heating. During the fall, thermal wind shear was strongest, because the cross‐isobath temperature gradient was acting in concert with the persistent cross‐isobath salinity gradient to enhance the cross‐isobath density gradient (i.e., warmer and fresher water inshore). During the winter, in response to surface cooling, the cross‐isobath temperature gradient reversed sign, reducing the cross‐isobath density gradient (i.e., cooler and fresher water inshore).Keywords: wind-driven circulation, dynamics, coastal oceanography, shelf-slope front, Middle Atlantic Bigh

Observations of tidal variability on the New England shelf

Observations from the Coastal Mixing and Optics experiment moored array, deployed from August 1996 through June 1997, are used to describe barotropic and baroclinic tidal variability over the New England shelf. The dominant M₂ tidal elevations decrease toward the northeast to a minimum over the Nantucket shoals (about 34 cm), and barotropic tidal current amplitudes increase strongly toward the northeast to a maximum over the shoals (about 35 cm s⁻¹). Estimates of the depth‐averaged M₂ momentum balance indicate that tidal dynamics are linear, and along‐shelf pressure gradients are as large as cross‐shelf pressure gradients. In addition, tidal current ellipses are weakly polarized, confirming that the dynamics are more complex than simple plane waves. The vertical structure of the M₂ currents decreases in amplitude and phase (phase lead near bottom) over the bottom 20 m. The M₂ momentum deficit near the bottom approximately matches direct covariance estimates of stress, confirming the effects of stress on current structure in the tidally driven bottom boundary layer. Baroclinic current variability at tidal frequencies is small (2 cm s⁻¹ amplitude), with a predominantly mode 1 vertical structure. High‐frequency (approaching the buoyancy frequency) internal solitons are observed following the pycnocline. The internal solitons switch from waves of depression to waves of elevation when the depth of maximum stratification is deeper than half the water column depth. Both low‐mode baroclinic tidal and high‐frequency internal wave energy decrease linearly with bottom depth across the shelf

Dynamics of mean and subtidal flow on the New England shelf

Current and hydrographic observations from the Coastal Mixing and Optics experiment moored array, deployed from August 1996 through June 1997, are used to describe the velocity variability and evaluate the dynamics of circulation over the New England shelf on timescales ranging from a few days to several months. Subtidal (days to weeks) current variability was polarized along-isobath and dominated by episodic bursts of westward flow. The along-isobath subtidal flow was primarily geostrophic and barotropic, and was correlated with large-scale along-coast wind stress fluctuations oriented 45°T (65° counterclockwise from the local isobath orientation). Subtidal nearsurface ageostrophic transport matched estimates of wind-driven Ekman transport; however, near-bottom ageostrophic transport was much larger than estimates of Ekman transport from bottom stress. Low-frequency (monthly and longer timescales) flow was generally westward and off-shelf at all sites and depths, with the strongest westward flow during the fall. Low-frequency along-isobath currents were primarily geostrophic with baroclinic and barotropic components of similar magnitude. Depth-averaged ageostrophic transport was quantitatively consistent with Ekman transport from wind and bottom stress. Measured bottom stress at both subtidal and low-frequency timescales was weak, nearly an order of magnitude smaller than the wind stress. Low-frequency fluctuations in the predominantly geostrophic along-isobath flow were attributable to variations in the cross-shelf density field associated with the seasonal cycle in surface heating. During the fall, thermal wind shear was strongest, because the cross-isobath temperature gradient was acting in concert with the persistent cross-isobath salinity gradient to enhance the cross-isobath density gradient (i.e., warmer and fresher water inshore). During the winter, in response to surface cooling, the cross-isobath temperature gradient reversed sign, reducing the cross-isobath density gradient (i.e., cooler and fresher water inshore).Keywords: Middle Atlantic Bight, Shelf-slope front, Wind-driven circulation, Dynamics, Coastal oceanographyKeywords: Middle Atlantic Bight, Shelf-slope front, Wind-driven circulation, Dynamics, Coastal oceanograph

Bottom boundary layer flow and salt injection from the continental shelf to slope

Austral winter oceanographic measurements from the northwest Australian continental shelf reveal salty water forming evaporatively inshore, moving across the wide shelf near the bottom and into the adjacent open ocean when the shelf edge alongshore flow is equatorward. The salt tongue is absent during more normal conditions, when the poleward Leeuwin Current is present. We hypothesize that the flow reversal enables shelf-wide bottom boundary layer (Ekman) transport and thus creates the shelf-edge convergence that accounts for the observed salt tongue. This flow is absent under sustained normal conditions because of buoyancy arrest in the bottom boundary layer

Alongshore currents and mesoscale variability near the shelf edge off northwestern Australia

Shelf break conditions and alongshore flow off northwestern Australia are studied during the strongly evaporative conditions of austral winter 2003. Present results, along with those of previous authors, confirm that a poleward, fresh Leeuwin current core is normally found near the shelf break. Salinity increases alongshore toward the southwest. Although there is no obvious shelf break front, there is a persistent offshore upward tilting of isopycnals in the depth range of 100–150 m. Repeated mesoscale surveys were made at the shelf edge during 8 days when the shelf break flow was equatorward. Waters offshore of the shelf break contain a rich baroclinic ageostrophic eddy field, with typically 10 km length scales, and the eddy patterns in the upper 60 m are uncorrelated with those below 90 m. The two depth horizons yield energy transfers from mean to eddy potential energy and appear to represent distinct finite amplitude instabilities on upper ocean and 180 m mean velocity cores, respectively. A linear stability model supports the existence of the two vertically separated instability types

Alongshore currents and mesoscale variability near the shelf edge off northwestern Australia

Shelf break conditions and alongshore flow off northwestern Australia are studied during the strongly evaporative conditions of austral winter 2003. Present results, along with those of previous authors, confirm that a poleward, fresh Leeuwin current core is normally found near the shelf break. Salinity increases alongshore toward the southwest. Although there is no obvious shelf break front, there is a persistent offshore upward tilting of isopycnals in the depth range of 100–150 m. Repeated mesoscale surveys were made at the shelf edge during 8 days when the shelf break flow was equatorward. Waters offshore of the shelf break contain a rich baroclinic ageostrophic eddy field, with typically 10 km length scales, and the eddy patterns in the upper 60 m are uncorrelated with those below 90 m. The two depth horizons yield energy transfers from mean to eddy potential energy and appear to represent distinct finite amplitude instabilities on upper ocean and 180 m mean velocity cores, respectively. A linear stability model supports the existence of the two vertically separated instability types.Keywords: shelf edge, Leeuwin, eddie